Drill chucks, which are constructed as so-called three jaw chucks, are common in a variety of useful embodiments. They can set a fixedly settable clamping force by corresponding actuation of the outer sleeve or they can be constructed so that at the beginning of operation self-tightening will take place. Examples of these kinds are described in EP 0054 774 B1, French Patent 1198531, German Patent 588 386 and German patent 37 44 589. Even embodiments with only two clamping jaws, each of which partly surrounds the bit shaft have already become known from U.S. Pat. No. 914,255. In all those drill chucks the clamping jaws are made completely of steel which is hardened at least in the region of the clamping surfaces.
A precise secure clamping of bits having a cylindrical shank, especially drill bits, is possible with the above-mentioned drill chucks. When clamped, the bit shaft, in consequence of the effect of the high surface pressure that arises at the essentially linear or strip-shaped clamping surfaces of the clamping jaws, is subjected to a slight elastic or possibly even plastic deformation which produces a fixed, rotation-resisting clamping of the bit shaft in the clamping jaws. This last-mentioned effect, however, no longer takes place in the case of drill bits which are entirely made of hardened metal or other completely-hard-metal tooling devices Which are today coming more and more into utilization. The hardness of the smooth and exactly cylindrical bit shafts of these bits, which is provided in view of the precision that is to be obtained, is so great that under the effect of the clamping force for which the drill chuck is designed, a highly loadable clamping of the bit shaft is no longer available. That has the result that upon the appearance of a mechanical loading peak there is the danger that the bit shaft will briefly slip with respect to the clamping jaws. That leads quickly to a release of clamping force and damage to the clamping jaws. Increasing the clamping force, by more or less strong manipulation of the positioning means after insertion of the bit shaft can therefore be of no help, for fear of producing plastic deformation of the clamping jaws that would make the chuck unusable.